The invention relates to digital coding of video signals and more particularly to differential pulse code modulation arrangements for reducing the transmission bit rate of digital video signals.
Pulse code modulation is commonly used to transmit video signals in digital form to take advantage of the features of a digital channel. As is well known in the art, digital conversion of a video signal requires sampling the video signal at a prescribed rate related to its bandwidth and forming a digital code for each video sample. In PCM coding, each sample is transformed into a fixed number of binary bits preset to accommodate the expected extremes of the signal samples. Video signals, however, contain redundant information so that the present signal value may be predicted from previous values. DPCM (differential pulse code modulation) coding encodes the difference between the present signal sample and a value predicted from past signal samples. Since the video signal is predictable, the difference values obtained by subtracting the predicted value for the present sample from the present sample results in a smaller dynamic range. Consequently, the number of bits of a DPCM code per sample applied to the digital channel is significantly lower than the number of bits of an equivalent PCM code and the needed transmission rate can be reduced.
U.S. Pat. No. 4,137,549, issued to Ichida et al. Jan. 30, 1979, discloses DPCM coding apparatus for encoding a composite color television signal in which the difference between a digital code corresponding to a signal sample and a predicted value for the signal sample signal is formed for each successive sample of the video signal. The difference between the present video signal sample and the predicted value therefor is quantized to one of a set of discreet values and the quantized signal is coded for transmission over a digital channel. The predicted value is formed by adding the quantized signal to the past predicted value and modifying the result for horizontal and vertical correlation of past values of the video signal. In this way, the range of the quantized difference signal is reduced so that a lower bit rate can be used for transmission of the coded video signal.
U.S. Pat. Nos. 4,536,880, 4,541,102, and 4,658,239 issued to Grallert, Aug. 20, 1985, Sept. 10, 1985 and Apr. 14, 1987, respectively, all disclose differential pulse code modulation arrangements in which the speed of calculation of the differential codes is increased by various means so that the time needed to process each sample is minimized to accommodate a high transmission bit rate. U.S. Pat. No. 4,541,102 describes an arrangement in which a quantizer is preceded by three cascaded subtractors to speed up calculation. U.S. Pat. No. 4,658,239 describes an arrangement having a pair of series connected arithmetic units connected to the input of a quantizer with multipliers interconnecting the quantizer to the arithmetic units to increase processing speed. U.S. Pat. No. 4,536,880 describes another DPCM system in which the number of calculating units is reduced to enhance processing speed. While the foregoing schemes are well adapted to improve speed to DPCM coding, the problem of a further reduction in the number of bits per code word is not considered. It is an object of the invention to provide an improved video coder using multiple prediction to lower the number of bits for coding a video signal.